This application claims the priority of German Application No. 197 28 577.5, filed Jul. 4, 1997, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a method for controlling the evaporator temperature of an air conditioner as a function of the outside dew point. Here, the dew point temperature of the intake air drawn in by the air conditioner (for air conditioning an interior) and the blown air temperature setpoints that are associated with one or more variably specifiable interior temperature setpoints for the intake air to be blown out into the interior are determined. In this context, the term "control" is to be understood in its broader sense and therefore includes both the case of pure control without feedback, as well as the case of regulation with feedback of the evaporator temperature's actual value to a control or regulating unit of the air conditioner. Determination of the intake air dew point temperature and blown air temperature setpoint is conventionally used, for example, to control (as a function thereof) a compressor that is located in the coolant circuit of the evaporator and can be switched on and off, or whose power can be regulated in such fashion that the desired evaporator temperature is obtained.
Such methods are particularly used for air conditioners of motor vehicles. Motor vehicle air conditioners of a modern conventional design have two basic operating modes for cooling operations. In a first operating mode without reheating, the evaporator temperature is controlled by the appropriate adjustment of the compressor power in such a manner that the intake air guided over the evaporator is only cooled to the point specified by an air conditioning regulator of the air conditioner in order to maintain an interior temperature that can be specified by the user. Therefore, only as much energy is used as is actually required for cooling the interior. However, no air drying takes place in this operating mode, so that precipitation on the vehicle window can occur in certain situations. In addition, under certain conditions, undesired evaporator odors in the form of drying odors can occur that are caused by a frequent change of the evaporator state, i.e., the state of the evaporator surface, between wet and dry when the evaporator temperature oscillates around the intake air dew point temperature.
The second operating mode is so-called reheating or counter-heating operation. In this mode, the compressor uses the evaporator to cool the intake air drawn in from the outside until shortly before the icing point at 0.degree. C. By means of a heating element that is located downstream from the evaporator in the intake air stream (in the form of a heat conductor for example), the intake air is then heated to the blown air temperature setpoint that is determined by the air conditioner regulator. In this operating mode, drying of the intake air takes place at the evaporator. This limits the tendency for condensation to form on the windows, and/or any windows that have condensation on them become clear once again. In addition, there are no excessive evaporator odors. However, energy consumption is significantly higher in this operating mode than in the first operating mode, since a higher cooling power is required and additional energy is also required for reheating.
Vehicle air conditioners with these two cooling modes are disclosed for example in German patent documents DE 36 24 171 A1 and DE 37 24 430 A1. The air conditioner described in DE 36 24 171 A1 incorporates a dew point temperature sensor system inside and/or outside that consists of a temperature sensor and a moisture sensor, with which an imminent undershoot of the dew point can be detected. This can be prevented by countermeasures, for which purpose the incorporation of an air blower is proposed in order to be able to blow a stream of air directed at the windows. Additionally, if necessary, the air to be blown out into the interior can be suitably conditioned by heating and/or dehumidification in order to prevent condensation on the windows. In DE 37 24 430 A1 the operating mode without reheating is selected as long as the outside temperature is higher than the inside room temperature setpoint.
The evaporator temperature is then set by so-called continuous cold regulation as a function of changes between the setpoint and the actual value of the interior temperature. If the outside temperature is between 0.degree. C. and the interior temperature setpoint, moderate reheating is selected in which the temperature of the reheating element is set as a function of the difference between the setpoint and the actual value of the interior temperature. At the same time, the evaporator temperature setpoint is adjusted so that it is always below the outside temperature by a constant preset value of 6.degree. C., for example. As a result of this limited drop in evaporator temperature, a sufficient amount of dehumidification is achieved with an acceptable level of energy consumption. The air humidity conditions (i.e, the intake air dew point temperature) in this type of evaporator temperature control are not taken into account.
It is known for example from German patent document No. DE 195 17 336 A1 to limit the setpoint for the evaporator temperature to a presettable maximum value to avoid odors.
It is an object of the present invention to provide a method for controlling the evaporator temperature of an air conditioner as a function of the outside dew point such that during cooling operation of the air conditioner no (or at most only comparatively minor) evaporator odors occur, while at the same time the consumption of energy is kept relatively low. In addition, the occurrence of undesired condensation effects such as precipitation on the windows in the case of vehicle air conditioners is prevented in reliable manner.
This and other objects and advantages are achieved by the method, according to the invention, for controlling the evaporator temperature of an air conditioner as a function of the outside dew point, especially a motor vehicle air conditioner in which the dew point temperature of the intake air drawn in by the air conditioner to air-condition an interior and the blown air temperature setpoint associated with one or more variably presettable interior temperature setpoints are determined for the intake air to be blown out into the interior. At the beginning of each cooling phase of air conditioner operation, a determination is made as to whether the evaporator is in a wet or dry state. (In other words, whether the evaporator surface is wet or dry). Then the evaporator temperature is adjusted to a setpoint that, within presettable temperature limits, with a wet evaporator state and a blown air temperature setpoint which is above the intake air dew point temperature, is chosen to be no higher than the intake air dew point temperature. Otherwise the evaporator temperature set point is chosen to be approximately as high as the blown air temperature setpoint.
This procedure has the advantage that the evaporator state which prevails at the outset is maintained (as much as possible) during the following cooling operation phase. Also, within the framework of this condition, the energy consumption for conditioning the intake air is kept relatively low. As noted before, if the evaporator state is initially wet and the blown air temperature setpoint is above the intake air dew point temperature, the maximum evaporator temperature is limited at the upper end to the intake air dew point temperature, so that the wet evaporator state is maintained. By suitable reheating, the intake air that is cooled at the evaporator to at least the intake air dew point temperature is again raised to the blown air temperature setpoint. On the other hand, if the evaporator state is initially wet but the blown air temperature setpoint is below the intake air dew point temperature, the evaporator temperature is adjusted to the blown air temperature setpoint, so that the wet evaporator state is maintained and no reheating is normally required.
If the evaporator is initially dry, the evaporator temperature is adjusted to the blown air temperature setpoint. With a blown air temperature setpoint that is above the intake air dew point temperature, this means that the dry evaporator state is maintained. It is only when the blown air temperature setpoint is below the intake air dew point temperature, and the evaporator is initially dry, that a single change of evaporator state from dry to wet during the subsequent cooling operation phase is unavoidable.
In general, the number of changes between dry and wet evaporator states during the operation of the air conditioner and hence the evaporator odors caused thereby are minimized. In addition, it is generally only in the case of initially wet evaporators and with a blown air temperature setpoint that is above the intake air dew point temperature that active reheating is required in the subsequent cooling operation phase. As a result, the overall energy consumption remains relatively low without the occurrence of any undesirable condensation effects.
In an advantageous embodiment of the present invention, the initial evaporator state is determined in a relatively simple manner by detecting and comparing the evaporator temperature with the determined intake air dew point temperature. When the initial evaporator temperature is higher than the intake air dew point temperature, an initially dry evaporator state is assumed. On the other hand, if the initial evaporator temperature is lower than the intake air dew point temperature, an initially wet evaporator state is assumed.
In still another advantageous embodiment of the present invention, the evaporator temperature is adjusted to a setpoint that is specified within presettable temperature limits as the minimum value of all blown air temperature setpoints as well as the difference between the actual and dew point temperatures of the intake air. In other words, the outside air minus a suitably selectable offset point. Apparently, with this comparatively simple method, only a few changes between the dry and wet evaporator states occur during the operation of the air conditioner. However, no interrogation of the evaporator state takes place in this case. Nevertheless, with this method, the previously mentioned advantageous embodiments can also be achieved in largely the same manner.
In very wet weather, the dew point temperature approximately corresponds to the actual outside temperature of the intake air, so that the corresponding difference is near 0.degree. C., and thus practically the maximum evaporator power is set in order to achieve the desired dehumidification of the air. In this case, the evaporator remains wet constantly. With high outside temperatures and dry weather, on the other hand, the evaporator temperature to be set can be determined by the minimum blown air temperature setpoint. As a result, depending on the operating conditions, the evaporator usually remains constantly wet or dry.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.